Saturday, 19 September 2015

Party tricks, again

It's not really a trick, you can't demonstrate the solution but the other blog entries under this label have been a bit tenuous too. Anyway it goes like this: you have a gun and you aim it at a target, that you're standing some distance away from, you pull the trigger and hit your target. Great the gun works as expected but what would you expect to happen if you weren't standing on the ground but on a flatbed rail carriage, on a railway line exactly parallel to the barrel of the gun you're aiming. That carrage is accelerated in exact synchronisation with the bullet when you pull the trigger. Of course that acceleration would be equivielent to about 60g or 70g so you'd be pureed into a gooey mess in the real world but lets suppose you're immune to that effect, what would happen to the bullet you fired, once it left the barrel?

If you answered that the bullet would just fall to the ground, congratulations you've won and this poser probably seems a bit nonsensical to you, until that is, you see how many people get it wrong, I was alarmed, it's easily the majority of people you ask the question of. The general consensus is that the bullet will hang in the air for a period, while it's "flying" and then fall to the ground. The fact that bullets don't fly, they're just objects in ballistic free fall, doesn't really seem to impact on that assumption, even if you use words of one syllable and pretty pictures to explain that fact. Now I confess, If I didn't know the answer and I was a little bit squiffy, like is generally the case at parties, I would probably have to think about it for few seconds, then there would be an, oh yeah of course, moment once I had it explained to me. You can divide the responses to the question, into several categories, which are:-

  1. Smarty: no problem, what a dumb question
  2. Slow but solid: works it out, eventually
  3. Dim: quick but with the wrong answer, accepts the correct answer readily
  4. Slow and stupid: wrong answer, may be reluctant to accept correct answer
  5. Intransigent idiot: quick or slow, will not accept the correct answer, even when everyone else in laughing
  6. Nutter: got it wrong, wont accept the correct answer, actively seeks to subvert the case for the correct answer, with coercion or social pressure, may succeed depending on their level of charisma or social status.


  1. If the carriage you were standing on moved at the same speed as the bullet from the moment you pulled the trigger, surely the bullet wouldn't have a chance to leave the gun?

    1. The bullet must leave the barrel, irrespective of the train's speed, because there is great deal of pressure between the bullet and the gun's breach face, from the burning propellant, usually over 40,000 psi in a rifle, somewhat less in pistol around 15,000 psi.

    2. Sorry, it's breech not breach.

    3. I'm not convinced. If the carriage moves forward at the same speed as the bullet (and therefore the gun also) at the point of pulling the trigger, the bullet would never leave the gun. The only way around that is if the bullet leaves the gun before the carriage moves, so you'd need to reword the question to allow for that. True, you do say that at the end of the question, but that seems to contradict your statement that "The carriage is accelerated in exact synchronisation with the bullet when you pull the trigger." Moving in "exact synchronisation" would include the pressure between the bullet and the gun's breech face.

    4. No movement of the train can negate pressure in the confined space of the barrel. The bullet must always accelerate from the breech end of the gun at the same rate and leave the barrel, regardless of any movement of the carriage. The key to the answer is that the carriage accelerates in exact synchronisation with the bullet, so if the bullet is accelerating at a particular rate and the carriage matches that rate, therefore the gun and its components also, what can be we infer about the carriage's acceleration? Bearing in mind that the bullet is accelerating through the barrel at the same rate as if the carriage were stationary.

    5. All the pressure does is propel the bullet. However, if the movement of the train is synchronised to the speed of the bullet, then the bullet can never leave the barrel - unless you allow for an initial movement of the bullet before the carriage is in synchronicity. If the carriage (and therefore the gun) moves at the same speed as the bullet at the same time the bullet begins to move - whatever that speed happens to be - then the bullet just can't leave the gun. Not unless, as I said, the carriage doesn't move until after the bullet has left the barrel.

    6. I think the problem stems from you inadvertently saying two different things at once. "When you pull the trigger" and "once it left the barrel". What I'm saying is that if the movement of the carriage matches the speed of the bullet when you pull the trigger, then the speed of the carriage is therefore linked to that initial propulsion. If you removed the "when you pull the trigger" part, and just had "once it left the barrel", then there's no problem. It's when you link the movement of the carriage to the movement of the bullet, then, by necessity, the carriage's speed matches that initial propulsion.

    7. That's a very interesting point of view.